Lethal yellowing destroyed about 300,000 coconut palms in Miami (Florida, USA) in less than fi ve years (McCoy, 1976). Previously, the disease killed more than 15,000 coconut palms in Florida, (USA).
The host list of palm species attacked by lethal yellowing is large and includes Phoenix dactylifera L.; P. canariensis Hort., and P. reclinata Jacq. (Thomas, 1974).
Developing fruits of the coconut start dropping from the palm followed by the formation of new inflorescences which rapidly become necrotic. These first symptoms are followed by a rapid and generalised yellowing, leading to the death of the palm (Figure 103).
In date palm the fronds become desiccated and grey-brown instead of becoming yellow. A soft rot of the growing point occurs, converting the meristematic area into a putrid, slimy mass. The crown topples from the palm, leaving a naked trunk.
The causal agent is a mycoplasma-like organism. It is believed that the pathogen is disseminated by wind-born arthropod vectors. Removal of diseased palms and their offshoots, quarantine measures, the use of tolerant types of palms and the treatment with antibiotics are the main control measures.
Nixon (1954)observed this disease in Al Hassa (Saudi Arabia). In Arabic, Al Wijam means poor or unfruitful. The disease is characterised by a retardation in terminal bud growth,and the whole crown of leaves formed after the occurrence of the disease have the rosetting symptoms. Newly formed leaves are reduced in size and marked by a faint narrow, yellow longitudinal line on the midribs (Figure 104). Leaves become chloritic and their life span is reduced. Death of leaves starts from the distal end and extends towards the base. Diseased spathes split open before their complete emergence and are reduced in size. The number and size of the bunches produced are also reduced year after year till the diseased palm fails to produce and dies.
Positive amplifi cation bands were obtained from DNA templates extracted from diseased tissue of date palm using the Polymerase Chain Reaction (PCR). These DNA tests offer basic support to the hypothesis that the cause of Al-Wijam disease is a Mycoplasma- like organism (Djerbi, 1999; personal communication).
Brittle leaves disease
Brittle leaves disease, also called “Maladie des Feuilles Cassantes” in French, was first observed in Nefta, Tozeur and Degache date plantations (Tunisia) and in Adrar, M’zab and Biskra (Algeria) (Djerbi, 1983).
Both adult and young palms including offshoots are attacked alike. A broad chlorotic striping of the pinnae followed by drying of the tip of the frond is the first symptom of this disease (Figures 105 and 106).
Yields drop signifi cantly as more fronds are affected and the retardation in terminal bud’s growth becomes evident. Leaves are shorter and of irregular size.
The causal agent remains unknown and no fungi or other pathogens were isolated. However, recent investigations with PCR showed that the causal agent seems to be a Mycoplasma-like Organism (Djerbi, 1999; personal communication).
Chemical analysis of date palm leaves and soils showed that concentrations of all nutrients in the tissue were higher in leaves of unhealthy palms. The exception was the concentration of manganese, which was ten times lower in the unhealthy palms (Djerbi, 1983). In addition, the conductivity and the phosphorus concentrations of the soil with diseased palms are higher than that of healthy ones. These results suggest that the areas affected by the disease have a build-up of major nutrients and salts as a result of irrigation, which have contributed to the high electrical conductivity. High pH and conductivity may have caused lack of manganese in the soil.
Quarantine measures seem to be the only means of limiting the spread of the disease. Since manganese is defi cient in unhealthy palms, this nutrient could be brought to these palms either by spraying or by injection. Djerbi (1983) found a gradient of susceptibility within Tunisian varieties even though they all seemed to be equally attacked.
Figure 103. Lethal yellowing in Florida on coconut palms (Cocos nucifera L.)
(Courtesy of Dr. McCoy)
Figure 104. First symptom of Al Wijam disease.